Q. We have been using liquid paint for our architectural aluminum extrusions for about 20 years, but have considered using powder coatings to reduce our VOC and save cost. We have seen some successful projects with powder, but we also hear stories about the UV resistance of powder when compared to high-performance liquid coatings and other performance issues. Can powder coatings meet AAMA 2604 and 2605 standards? What are the strengths and limitations of powder versus liquid coating? B.A.
A. There are two critical qualifiers that are important to the discussion. First, the surface has to have the right treatment to ensure good adhesion and corrosion resistance. For architectural aluminum, this means using chrome, chromate, or a high-quality, non-chrome conversion coating. Second, the coating must be a high-quality material with the necessary properties to meet the specification. If the surface treatment is good and the coating is a superior coating with the needed properties, liquid and powder can both be effective.
The AAMA standards describe treatments in some detail and recommend minimum performance levels and rigorous testing to meet the specification. These specifications include minimum coating weights for the chrome and severe adhesion testing to confirm success.
AAMA 2604 is a five-year standard for outdoor weathering that defines a limited amount of gloss loss and color loss. A well-engineered polyester powder will meet this standard. These products usually are super-durable resins. It is very important to understand that the resin must have complimentary pigments and additives to achieve the standard. Just using a super-durable resin is not enough; it must be formulated with the right ingredients to meet the standard. Also, not all colors can be formulated to meet the standard due to the vulnerability of their pigments to ultra-violet (UV) light. Still, generally speaking, properly treated aluminum surfaces and good super-durable polyester can meet AAMA 2604 with one coat of powder.
AAMA 2605 is a 10-year standard that requires more sunlight resistance and better corrosion resistance. The increased UV resistance is a challenge to organic coatings of all types. Most products used for 2605 are fluoro-polymer products and can be formulated in liquid or powder. They do not provide the low-VOC advantage associated with standard organic formulas.
Powder Strengths
- Powder is a one-coat process with performance that is equal to some two-coat liquid systems.
- Powder is much better for environmental
- compliance.
- Powder typically uses less energy.
- Powder often has better mechanical properties and stain resistance compared to liquid.
- No runs, drips or sags.
- No mixing.
Powder Limitations
- Standard organic powders do not meet AAMA 2605.
- Formulation for 2605 reduces the environmental advantage substantially.
- Not all colors will work for 2605.
Some closing comments:
- Some companies have stated publicly that they meet AAMA 2605 with a three-stage washer. This is not possible, as a three-stage cannot deliver the needed chrome coating weight as defined by the specification.
- Some companies advertise that they meet the AAMA specifications because they use a qualified powder. This is misleading, because they must meet the testing requirements of the specification or they are not AAMA qualified.
- Proper application and cure are critical to make sure the coating covers the entire surface at the correct thickness and that it is fully cured.
- The design of the part has a profound impact on performance. If the part traps water, has bear edges or other design defects, it can fail no matter how it is coated.
- Primer can radically improve the longevity of any coating.
You should do your homework on powder and make sure you understand how to use it properly. If you do, you should be very satisfied with powder as an outdoor, AAMA-compliant coating.
